The pattern of lights along a buoyed channel as seen by the navigator of a ship proceeding along the channel is similar to the pattern of street lamps seen by a motorist driving along a lighted street, except that the buoy-lights are below eye-level rather than above. The distant lights appear to move very slowly, while the apparent motion of near lights increases rapidly as they pass out of the driver's field of view on either his left or his right side. The form of the changing picture that should be presented on a screen, in front of an observer, in order to simulate the changing navigational situation, corresponds to the pattern that he would actually see if he were standing on the ship's bridge (at a particular height above the water) at a particular, but alterable, point in the channel.
It will be apparent that the pattern of lights may comprise not only buoy lights, but also the lights of other vessels, shore lights, lighthouse beacons, etc. The term navigation light used hereinafter is intended to cover all of these forms of lights.
A particular characteristic of buoy light (as opposed to a ship's navigation light) is that it does not show continuously but shows for, say, 1 second in every 10 seconds. Alternatively, it may be showing for most of the time but be extinguished for, say, 1 second in every 10 second. A buoy operating in the former manner is known as a "flashing" buoy, and a buoy operating in the latter manner is known as an "occulting" buoy.
Some buoys show a white light, others show a red, some green, and a lighthouse may show a sequence of flashes using two or more colours. (For simplicity of description "white" is deemed to be a colour). It would be advantageous therefore, that each light spot projected onto the screen be controllable so that it can appear intermittently and in either of two or more colours.